In the arts relating to apparatus for the production and contouring of sheet material, such as rolling mills, paper making machines, rubber and plastic calenders, sheet coaters, and the like, it is of highly significant concern to control the sheet material so as to maintain one or more of its characteristics within precise limits. The production and/or contouring of sheet material within close tolerances is critical both from the standpoint of quality -- optimum durability, reliability, uniformity, and appearance -- and from the standpoint of reducing the costs associated with production -- minimized use of raw material, minimized production of material which does not meet specifications, and minimized non-productive machine and manpower time. It is well known in these arts, therefore, to measure for the purposes of control such characteristics as thickness, weight per unit area, density, moisture content, resistivity, and other physical or chemical characteristics of the sheet.
A typical form of such measurement utilizes a gauge mounted for a scanning traversal of the sheet material to measure the characteristic at a plurality of points across the sheet in order to provide a transverse profile of the sheet with respect to the characteristic. From the profile information, an average value of the characteristic across the strip or a portion thereof may be calculated. The average value may be compared to a desired target, and any deviation from the target may be used as the basis of a feedback change in the process to reduce the deviation to zero. U.S. Pat. No. 3,244,881 issued Apr. 5, 1966 to C. W. Hansen et al., and U.S. Pat. No. 3,307,215 issued Mar. 7, 1967 to G. R. Gerhard et al., the latter being assigned to the assignee of the present application, are representative of this type. In the first mentioned patent, wherein a profile of instantaneous sheet thickness across the sheet width is recorded on a strip recorder, an accurate cross direction profile is not obtained because the measurements, due to the fact that they represent merely an instantaneous sampling of thickness and are obtained along an oblique line with respect to the cross direction, are affected by machine direction variations. In the latter patent, in one embodiment of which the profile consists of an average thickness measurement taken across each of several transverse zones, the influence of machine direction variations is somewhat reduced; however, the measurements are also taken along an oblique line. U.S. Pat. No. 3,552,203 issued Jan. 5, 1971 to E. J. Freeh, assigned to the assignee of the present application, describes an embodiment in which thickness averages are accumulated for each of a plurality of zones over a number of scans by a scanning gauge, and a process of interpolation provides an estimation of average thickness at points across the width other than at the centers of the zones, which are assumed to be at an average zonal thickness value. However, the zonal thickness measurements are again taken along an oblique line. Moreover, the interpolation process does not provide an estimated profile between the centers of the outer zones and the edges of the sheet.
While a gauge of the continuously scanning type, capable of providing measurements of the sheet characteristic at a large number of points across the sheet width or a portion thereof has general utility in production control systems of these arts, certain processes demand an alternative form of gauge. For example, where a substantially instantaneous profile of the characteristic is required, a scanning gauge, because of the often substantial period of time needed in which to traverse the sheet, may be inappropriate. A scanning gauge, moreover, can provide a profile of the sheet characteristic only along a line which is oblique to the cross direction of the strip, so that such a profile necessarily reflects components of both cross direction and machine direction. One might, of course, provide a large number of fixed gauges across the sheet width, from whence to derive a cross direction profile; however, the hardware and maintenance costs associated with such a system would be prohibitive.
The present invention offers a unique and accurate alternative to a scanning gauge for the purposes of determining a profile of a material strip with respect to a particular characteristic. Moreover, it offers significant advantages over a scanning gauge system, in that it is capable of providing a substantially instantaneous profile consisting of measurements taken along a line exactly normal to the machine direction. In this way, machine direction variations are practically eliminated as factors affecting a cross direction profile of the material sheet.
The present invention has the further advantage of being adapted for use in a control system in which the use of a scanning gauge would be impractical for reasons of space limitations, difficulties in gauge alignment and calibration, gauge geometry, etc.
Thus the present invention gives an accurate estimation of the profile of a material sheet, which has potential applicability to many types of sheet processing control systems, as will be apparent from a reading of the description which follows.